The present invention relates to a semiconductor device and a method for manufacturing the same, and more particularly to a method for forming a gate insulating film used in a MIS transistor.
A MOS transistor is a typical MOS device. For example, in a complementary MOS (CMOS) transistor, or the like, a high speed driving transistor, which is required to have a gate insulating film of a relatively thin thickness, and a high breakdown-voltage transistor, which is required to have a gate insulating film of a relatively thick thickness for handling input/output signals of a relatively high voltage, are formed on a single semiconductor substrate.
The high speed driving transistor is required to have a gate insulating film having a thickness of about 1 nm to 3 nm, while it is strongly required to have a high reliability in resisting against dielectric breakdown and to have a low leakage current.
A CMOS transistor employs a so-called “dual gate structure”, in which the gate electrode of the P-channel transistor is a P-type gate electrode obtained by using boron (B) as a dopant, and the gate electrode of the N-channel transistor is an N-type gate electrode obtained by using phosphorus (P) as a dopant. Boron, being a P-type dopant, has a larger diffusion coefficient than that of phosphorus, being an N-type dopant, whereby during a heat treatment after the transistor is formed, boron diffuses through the gate insulating film of the high speed driving transistor to reach the channel region. The diffusion of boron is called “boron penetration”, and causes various problems in the transistor such as a substantial variation in the threshold voltage and a deterioration of the driving ability. The boron penetration is, of course, more pronounced as the thickness of the gate insulating film is reduced, and is particularly pronounced when silicon dioxide (SiO2) is used for the gate insulating film.
Furthermore, reducing the thickness of the gate insulating film also causes an increase in the gate leakage current through the gate insulating film. Again, where silicon dioxide is used for the gate insulating film, the conduction mechanism thereof is a Fowler-Nordheim tunneling current if the thickness is 3.5 nm or more, and the direct tunneling current becomes dominant if the thickness is 3.5 nm or less. The gate leakage current increases by an order of magnitude for every 0.2 nm decrease in the thickness of the gate insulating film. If the thickness of the gate insulating film is set to be 2.6 nm or less, the gate leakage current is no longer negligible.
As described above, if a thermal oxide film is used for the gate insulating film, it is no longer possible to suppress the boron penetration and the gate leakage current. In view of this, an oxynitride film into which nitrogen is introduced has been used as a gate insulating film.
A conventional method for forming a gate insulating film of a MOS semiconductor device using a silicon oxynitride film will now be described with reference to the drawings.
FIG. 12A to FIG. 12C are cross-sectional views sequentially illustrating the steps of the conventional method for forming a gate insulating film.
First, a device isolation region 102 that partitions a plurality of device forming regions from one another is formed in an upper portion of a semiconductor substrate 101 made of silicon, and then a first gate oxide film 103A made of a thermal oxide film having a thickness of about 7.5 nm is formed entirely across the upper surface of the semiconductor substrate 101. Then, a resist pattern 104 having an opening in a second region 202 is formed on the first gate oxide film 103A, and then a portion of the first gate oxide film 103A that is included in the second region 202 is etched away using the resist pattern 104 so that the second region of the semiconductor substrate 101 is exposed, thereby obtaining a structure as illustrated in FIG. 12A.
Then, as illustrated in FIG. 12B, the semiconductor substrate 101 is subjected to a heat treatment so as to form a second gate oxide film 105A made of a thermal oxide film having a thickness of about 2.6 nm in the second region 202. In this process, the thickness of the first gate oxide film 103A increases.
Then, as illustrated in FIG. 12C, the semiconductor substrate 101 is subjected to a heat treatment in an oxynitriding atmosphere made of nitrogen monoxide (NO) at a temperature of 900° C. for 30 seconds to several ten minutes so as to introduce nitrogen into the first gate oxide film 103A and the second gate oxide film 105A, thereby obtaining a first gate oxynitride film 103B and a second gate oxynitride film 105B, respectively. Note that other than nitrogen monoxide (NO), dinitrogen monoxide (N2O) or, though rarely, ammonia (NH3) may be used in the oxynitriding process using a heat treatment.
When nitrogen monoxide (NO) is used, the oxynitriding process increases the thickness only by 0.3 nm or less. In contrast, when dinitrogen monoxide is used, it is required to perform an oxynitriding process under a high temperature of about 1000° C. to 1150° C. for several ten seconds to several ten minutes, whereby the oxynitriding process with dinitrogen monoxide (N2O) increases the thickness by a substantial amount of up to several nanometers. Therefore, with dinitrogen monoxide (N2O), care should be taken for the process.
FIG. 13A and FIG. 13B are each a nitrogen concentration profile in a gate oxynitride film which has been oxynitrided by using an oxynitriding atmosphere made of nitrogen monoxide (NO), wherein FIG. 13A is for the first gate oxynitride film 103B and FIG. 13B is for the second gate oxynitride film 105B. As illustrated in FIG. 13B, in the second gate oxynitride film 105B having a thickness of 2.6 nm, the nitrogen atom peak is and to make it possible to reduce the gate leakage current while preventing dopant atoms from diffusing from a gate electrode into a substrate through a gate insulating film having a thickness that is so reduced that a direct tunneling current can flow therethrough.
The present inventor has conducted various researches to make it possible to suppress the penetration or diffusion of boron through a thinned gate insulating film while reducing the gate leakage current. As a result, it has been found that it is preferred that the nitrogen concentration in the gate insulating film is sufficiently high with a broad nitrogen distribution in the insulation film and that a low-temperature process is used for an improved control of the thickness of the thin film. Moreover, it has also been found that in order to obtain an increased nitrogen concentration in an oxynitriding process using nitrogen monoxide or dinitrogen monoxide, it is necessary to increase the heat treatment temperature of the oxynitriding process, which is not suitable for the formation of a very thin gate insulating film.
Therefore, in order to prevent the penetration of boron through a gate insulating film and to increase the dielectric constant and the refractive index by increasing the nitrogen concentration, thereby reducing the gate leakage current, it is necessary to form, as a thin gate insulating film, an oxynitride film that has a broad nitrogen distribution with a nitrogen peak concentration over 10 atm %.
Specifically, a semiconductor device of the present invention includes: a gate insulating film formed on a semiconductor substrate; and a gate electrode formed on the gate insulating film, wherein nitrogen is introduced into the gate insulating film and a nitrogen concentration distribution thereof has a first peak near a surface of the gate insulating film or near a center of the gate insulating film in a thickness direction.
With the semiconductor device of the present invention, nitrogen is introduced into the gate insulating film, and the nitrogen concentration distribution thereof has the first peak near the surface of the gate insulating film or near the center of the gate located near the interface between the second gate oxynitride film 105B and the semiconductor substrate 101. The peak concentration is about 4 atm % at maximum, through it varies depending on the oxynitriding temperature. Note that also when the oxynitriding process is performed by using dinitrogen monoxide (N2O), the nitrogen concentration profile is as that shown in FIG. 13B, and the peak concentration is, at best, 1 atm %.
The second gate oxynitride film 105B obtained by the conventional oxynitriding process has a nitrogen concentration profile and a nitrogen concentration peak as shown in FIG. 13B, whereby boron ion implanted into the p-type gate electrode of the P-channel transistor diffuses through the second gate oxynitride film 105B relatively easily, though it depends on the heat treatment temperature, and reaches the channel region in the semiconductor substrate 101. The diffusion of boron is of course suppressed as compared with a gate oxide film made only of silicon dioxide. However, when the thickness is reduced so much as in the second gate oxynitride film 105B, it is not possible substantially prevent the diffusion of boron with a nitrogen concentration profile in which the nitrogen peak concentration is only about 4 atm % and the peak is located near the interface with the semiconductor substrate 101. This is the first problem in the prior art.
Furthermore, with such a silicon oxynitride film, in which the nitrogen concentration is only about 4 atm % and the nitrogen atoms are localized near the substrate interface, the nitrogen content of the film as a whole is not sufficient to change the dielectric constant and the refractive index of silicon dioxide (SiO2), and thus it is certainly not expected to be sufficient to provide an increase in the electric capacitance or a reduction in the gate leakage current. This is the second problem in the prior art.